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NCP1253 Datasheet(PDF) 13 Page - ON Semiconductor

Part No. NCP1253
Description  Current-Mode PWM Current-Mode PWM Power Supplies
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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NCP1253 Datasheet(HTML) 13 Page - ON Semiconductor

 
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NCP1253
http://onsemi.com
13
Fsw
VFB
VCS
VFB
65 kHz
26 kHz
350 mV
Vfold
3.4 V
Vfold
3.4 V
0.8 V
[0.36 V
FB
Vfreeze
[0.25 V
1.05 V
1.5 V
1.5 V
max
min
max
min
Vfold,end
Frequency
Peak current setpoint
Fsw
VFB
VCS
VFB
65 kHz
26 kHz
350 mV
Vfold
3.4 V
Vfold
3.4 V
0.8 V
[0.36 V
FB
Vfreeze
[0.25 V
1.05 V
1.5 V
1.5 V
max
min
max
min
Vfold,end
Frequency
Peak current setpoint
Figure 27. By Observing the Voltage on the Feedback Pin, the Controller Reduces its Switching Frequency for an
Improved Performance at Light Load
Auto−recovery Short−Circuit Protection
In case of output short−circuit or if the power supply
experiences a severe overloading situation, an internal error
flag is raised and starts a countdown timer. If the flag is
asserted longer than 100 ms, the driving pulses are stopped
and VCC falls down as the auxiliary pulses are missing.
When it crosses VCC(min), the controller consumption is
down to a few
mA and the VCC slowly builds up again thanks
to the resistive starting network. When VCC reaches
VCCON, the controller attempts to re−start, checking for the
absence of the fault. If the fault is still there, the supply enters
another cycle of so−called hiccup. If the fault has
disappeared, the power supply resumes operations. Please
note that the soft−start is activated during each of the re−start
sequence.
1 vcc
2 vdrv
3 ilprim
500u
1.50m
2.50m
3.50m
4.50m
time in seconds
445m
1.41
2.38
3.35
4.32
−8.13
−2.12
3.89
9.90
15.9
−11.5
−2.72
6.05
14.8
23.6
2
1
3
()
cc
Vt
()
DRV
Vt
()
p
L
It
SS
1 vcc
2 vdrv
3 ilprim
500u
1.50m
2.50m
3.50m
4.50m
time in seconds
445m
1.41
2.38
3.35
4.32
−8.13
−2.12
3.89
9.90
15.9
−11.5
−2.72
6.05
14.8
23.6
2
1
3
()
cc
Vt
()
DRV
Vt
()
p
L
It
SS
Figure 28. An Auto−Recovery Hiccup Mode is Entered in Case a Faulty Event Longer Than 100 ms is
Acknowledged by the Controller
Ramp compensation
The NCP1253 includes an internal ramp compensation
signal. This is the buffered oscillator clock delivered during
the on time only. Its amplitude is around 2.5 V at the
maximum duty−cycle. Ramp compensation is a known
means used to cure sub harmonic oscillations in
CCM−operated
current−mode
converters.
These
oscillations take place at half the switching frequency and
occur only during Continuous Conduction Mode (CCM)
with a duty−cycle greater than 50%. To lower the current
loop gain, one usually injects between 50% and 100% of the
inductor downslope.


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